The present disclosure relates to a system and method for making direct reduced iron. Direct reduced iron (DRI), sometimes called sponge iron, is a commercial product widely used as a source material for making steel. The conventional techniques for making steel involve the use of an electric arc furnace (EAF) or a basic oxygen furnace (BOF). DRI is typically higher in iron units than taconite pellets and other sources of iron, and can be used as a partial substitute for scrap in the production of steel by EAF.
DRI is formed from beneficiated iron ore, such as taconite pellets. For example, taconite has been mined and crushed, and the iron containing portions magnetically separated from the non-iron containing portions to form a beneficiated product higher in iron content than mined taconite. The beneficiated iron ore portion may be formed into pellets by pelletizing, and heated in a linear hearth furnace in the presence of reducing agent (e.g., carbonaceous material) to a temperature below the melting point of iron using natural gas or coal, to promote the reduction of iron ore to metallic iron. DRI is typically above 90% metallic iron with the remainder gangue.
In the process to make DRI, the beneficiated and pelletized iron oxide containing material is moved through a furnace mixed with a reducing agent, such as coal, coke, or another form of carbonaceous material. A desulfurizing agent, such as limestone or dolomite, is also typically added. The carbon of the reducing agent and the oxygen of the iron oxide material react chemically in the reducing zone of the furnace, thereby partially reducing the iron oxide to form metallic iron. This, and other traditional reducing processes, are used to create the DRI.
DRI is difficult to transport because DRI and DRI fines are highly reactive with oxygen in air and moisture. Moisture, in particular, reacts with the iron forming FeO and H2. The DRI being sponge iron has many voids making it porous in nature. The porous nature of DRI also means that it has low compressive strength, and handling of DRI generates surface fines. Additionally, when the DRI is stored, for example in the hold of a ship during transportation, some of the pellets have been prone to disintegrate under the weight of pellets above them further generating fines and small particles. The DRI fines and small particles increased the ability for reaction with moisture and oxygen around it. Additionally, the rough surface characteristics of the DRI pellets produce particulate matter and other fines having a high surface area, which also promoted the likelihood of the DRI reacting with oxygen. Such particulate matter and fines typically are produced throughout the storage and transportation of the DRI, making it difficult to transport DRI over long distances and to store DRI for long periods.
The porous, low internal strength, and flakey nature of DRI all increase the surface area of the nodule that is exposed to an oxidizing atmosphere and/or moisture, resulting in substantial and rapid oxidation and rusting. The reactions that occur during DRI oxidation produce heat and hydrogen making DRI susceptible to overheating and combustion. Increases in temperature in containers storing DRI, in which air is free to circulate, can reach 1200° F. Such combustion causes fires in the holds of ships during transportation of DRI and in the clam buckets of cranes when unloading DRI. These risks have substantially increased the cost of DRI delivered to a steel plant because of the losses during transportation and storage. Due to the difficulties and risks associated with transporting DRI, production of DRI has with a few exceptions been generally located near the steelmaking facilities and near the time of use in steelmaking, rather than in more economical locations and times.
Consequently, various techniques have been used in the past to passivate DRI to reduce the risks associated with its pyrophoric properties and improve its compressive strength. However, despite various attempts there still remains a need for an economic and efficient way of passivating DRI so it can be safely transported over long distances in bulk and stored. A strong, stable and pyrophobic product would enable the safe transport and storage of DRI, dramatically increasing its usefulness and effectiveness in steelmaking.
Presently disclosed are a method and system for making processed DRI. The method comprises assembling a rotatable chamber having an internal screen capable of supporting DRI during tumbling within the chamber, with at least one opening in the chamber adapted to permit fines to exit the chamber during tumbling, and delivering DRI into the rotatable chamber and rotating the chamber to tumble the DRI on the screen in the chamber to remove fines from the DRI. The screen may have a mesh size between ⅛ and ¼ mesh. The method may also include evacuating fines removed from the DRI through the opening or openings in the rotatable chamber during rotation of the chamber, and may include evacuating fines removed from the DRI during tumbling. The DRI may be tumbled in the rotatable chamber at between 20 and 50 or between 20 and 40 revolutions per minute for a residence time of at least 10 minutes in the chamber to produce the desired removal, which is manifested by the DRI having a polished appearance.
The method and system for making processed DRI may also include assembling rotatable rollers downstream of the rotatable chamber adapted to rotate the DRI and applying oil to the processed DRI. Spray nozzles may be positioned and adapted to deliver oil to the processed DRI rotating on rollers. The oil may be mineral oil, and may include oleic acid. The oil may be heated prior to delivery of the oil to the processed DRI.
The method and system for making processed DRI may further comprise applying a material adapted to increase the compressive strength of the DRI to the processed and oiled DRI. The material may include limestone, such as a limestone based binder.
A method and system for making processed DRI is also disclosed that comprises assembling a rotatable chamber having a feed end and an exit end, and having an internal screen capable of supporting DRI during tumbling as the DRI moves through the rotating chamber from the feed end to the exit end and having at last one opening along the chamber to permit fines removed from the DRI during tumbling to exit the chamber, delivering DRI to the rotatable chamber through the feed end and rotating the chamber to tumble the DRI on the screen in the chamber while the DRI moves through the chamber from the feed end to the exit end while removing fines from the DRI, and removing processed DRI from the discharge end of the rotatable chamber.
Also disclosed is a processed DRI material having a surface roughness (Ra) of less than 1.5 μm. Alternatively, the processed DRI material may have a surface roughness (Ra) of less than 1.0 μm, or less than 0.75 μm. The processed DRI material may comprise pellets, and may have a tumble index of greater than 98.5%+¼ after 200 revolutions. In other examples, the processed DRI material may be coated with oil, and the oil may comprise mineral oil and oleic acid. The processed DRI material may also comprise a binder, such as limestone, to increase the compressive strength of the DRI material.
Also disclosed is a processed DRI material having a surface roughness (Ra) of less than 1.5 μm made by the steps of assembling a rotatable chamber having a feed end and an exit end, and having an internal screen capable of supporting DRI during tumbling as the DRI moves through the rotating chamber from the feed end to the exit end and having at last one opening along the chamber to permit fines removed from the DRI during tumbling to exit the chamber, delivering DRI to the rotatable chamber through the feed end and rotating the chamber to tumble the DRI on the screen in the chamber while the DRI moves through the chamber from the feed end to the exit end while removing fines from the DRI, and removing processed DRI from the discharge end of the rotatable chamber.